1. Field of the Invention
The present invention relates to an optical transmission system using optical fiber.
2. Description of the Related Art
In the field of optical transmission systems, the development of larger-capacity, longer-distance systems is under way. To increase the transmission capacity, methods of increasing the bit rate are being studied along with wavelength multiplexing techniques. On the other hand, longer transmission distances can be accomplished by introducing optical amplifiers. Optical amplifiers are being developed as post-amplifiers for increasing transmitter power, as preamplifiers for boosting receiver sensitivity by raising input power, or as in-line amplifiers for repeaters. With the introduction of optical amplifiers, the allowable level difference between fiber input and output has been increased, expanding the range of allowable fiber loss.
On the other hand, the use of optical amplifiers has introduced a new problem of nonlinear effects because of increased optical input levels to the fiber. One is self-phase modulation (SPM), due to the optical Kerr effect (refractive index varies depending on light intensity), which causes frequency (wavelength) shifts in the rising and falling portions of a signal light pulse. In that case, even if the wavelength range of signal light before transmission is narrow, the signal light spreads out in wavelength range during transmission, and at the same time, the received waveform changes greatly because of the effect of chromatic dispersion. In other words, the upper limit of transmission optical power is determined by considering such effects.
Further, the velocity of the light propagating through a fiber depends on the wavelength of the light (this is called the chromatic dispersion of the fiber). Accordingly, light pulses containing a range of wavelengths tend to spread out or contract in pulse width as they travel along a fiber. Therefore, in an optical transmission system, the received waveform after transmission through a fiber is distorted because of the chromatic dispersion and, depending on the degree of the distortion, transmission errors occur. Chromatic dispersion can thus limit the transmission distance.
Previously, transmission degradation by fiber chromatic dispersion has been avoided by selecting a light source having a narrow wavelength range. However, as the bit rate increases up to 10 Gb/s, the problem of the fiber nonlinear effects arises, causing a situation where the transmission degradation cannot be avoided by simply selecting a light source having a narrow wavelength range.
In view of this situation, it has been proposed to compensate for transmission characteristics by using transmitter prechirping as well as selecting a light source having a narrow wavelength range. Transmitter prechirping is a technique for causing chirping in light pulses in the transmitter. There are two types of chirping: blue chirping that causes the wavelength to shift to the longer wavelength side at the rising of an output pulse and to the shorter wavelength side at the falling thereof, and red chirping that causes the wavelength to shift to the shorter wavelength side at the rising of an output pulse and to the longer wavelength side at the falling thereof, and the type of chirping is selected depending on the fiber mainly used in the transmission channel. Japanese Unexamined Patent Publication No. 4-140712 describes how transmission characteristics can be improved by applying blue chirping (chirping parameter .alpha. is negative) when the fiber has positive chromatic dispersion with respect to the signal light, and red chirping (chirping parameter .alpha. is positive) when the fiber has negative chromatic dispersion. That is, when the blue chirping is combined with positive chromatic dispersion or the red chirping combined with negative chromatic dispersion, the falling portion of a light pulse travels through the fiber faster than the rising portion thereof, which has the effect of contracting the light pulse. In this case, since the fiber's dispersion value is proportional to its length, a dispersion compensator is inserted in series with the fiber to make the overall dispersion value of the transmission channel match the amount of prechirping in the transmitter.
The type of fiber currently most popular and widely installed is the single-mode fiber (SMF) which has zero dispersion wavelength in the 1.3 .mu.m band. This is because, in the case of a fiber with a relatively simple structure consisting of a uniform clad and core, the longest wavelength where zero dispersion can be achieved is 1.3 .mu.m, and at longer wavelengths, zero dispersion can be achieved only by using a dispersion-shifted fiber (DSF) which is expensive and complex in structure, and also because of this, the 1.3 .mu.m band, where fiber attenuation is considered low, has traditionally been used as the band for signal light wavelength. However, to extend transmission distance with the introduction of optical amplifiers, it becomes necessary to use signal light in the 1.5 .mu.m band where erbium-doped fibers as optical amplifiers have gain regions and where fiber attenuation is considerably lower. If signal light at 1.5 .mu.m is transmitted through an SMF whose zero dispersion is in the 1.3 .mu.m band, dispersion is positive. In the prior art, therefore, it has been attempted to apply blue chirping (chirping parameter .alpha. is negative) to signal light in the transmitter, to try to improve the transmission characteristics by the combination with the positive dispersion of the fiber.
However, as will be described in detail later, in a computer simulation carried out considering the SPM, it has been found that if the chirping parameter .alpha. is negative, there arises the problem that many types of dispersion compensators that are expensive have to be prepared in advance for a variety of the fiber lengths since the range of the dispersion compensation amount that satisfies the desired transmission characteristic is narrow.